Less than perfect signal transmission line characteristics cause intersymbol interference in transmitted data signals. The extent of the interference depends upon the length and the impulse response characteristic of the transmission line. If signal trace displays of multiple successive data symbols are superimposed, e.g., as in an oscilloscopic display, the resulting composite display defines a generally elliptical figure with somewhat pointed ends on the major, and usually horizontally displayed, axis including both foci and having a central opening of a vertical size that is a function of the extent of intersymbol interference contained in the signal. Such a display is commonly said to be an eye pattern, or eye, because of its general similarity to the frontal view of the human eye with the eyelid open. Signal detection error rate is usually lowest for properly sampled signals exhibiting the most widely open eye and for which detection sampling takes place in the data signal bit time phase of widest eye, i.e. the phase of the minor axis of the ellipse defined by the eye opening.
Intersymbol interference in the form of extension of the trailing edge of a data symbol into the symbol time of at least one following symbol is said to be postcursor interference, and interference in the form of extension of the leading edge of a data symbol into symbol times of at least one preceding symbol is said to be precursor interference. In either a switched or a private line digital system for telecommunication, hereinafter simply "digital transmission circuit," there is a relatively wide range of circuit characteristics that may be encountered in establishing a digital connection for, e.g., a special service circuit. In addition, those digital transmission circuits are usually required to transmit high speed data, e.g., data at more than 100 kilobits per second (kbps), as compared to low speed data, e.g., at about 4.8 kbps. High speed data is therefore subject to greater intersymbol interference, especially that due to bridged taps on a line such as are commonly present on data circuits. Consequently, the impulse responses of circuits included in a digital transmission circuit differ widely, and unpredictably, in configuration; and often exhibit substantial lack of symmetry about a time of maximum amplitude. An equalizer employed in such a facility, must therefore, be able to equalize a wide range of circuits in order that a large inventory of different equalizer types not be required.
Decision feedback equalizers are known in the art to be advantageous for dealing with intersymbol interference in multilevel digital data circuits because they are relatively simple in that signal multiplications can be accomplished by relatively simple logic, e.g. additions, subtractions, and shift circuitry to accommodate multiplications by 2, rather than by complex analog or digital multipliers. Decision feedback equalizers, in general, include arrangements to make data decisions that quantize the received signal amplitude into 1 of N possible states, where N corresponds to the number of distinct levels which must be decoded, and produce a corresponding output. They also include feedback of the same output through an estimation filter, such as a multitap, adaptive, transversal filter, designed to extract postcursor interference components from the output and apply them in a canceling relation to the equalizer input. In the specific case of a binary system considered hereinafter, e.g., ONE-ZERO data represented by +1 and -1 symbols, the decision feedback equalizer must make a binary ONE-ZERO decision and produce a corresponding output. Since the estimation filter receives a binary type of input signal rather than a multilevel input signal, the tap coefficient update and convolution logic can be relieved of multipliers and use addition and subtraction circuits which are simpler to implement.
U.S. Pat. Nos. 4,170,758 and 4,283,788 to G. Tamburelli teach such an equalizer configuration, and they also show arrangements for coupling the equalizer output through multiple sections, or cells, of filters for extracting precursor components and combining them in subtractive relation with differently delayed predecision versions of the signal to compensate for the precursor aspect of the signal. The number of cells required depends upon the extent of precursor included in the impulse responses of the circuits being equalized as shown in a Tamburelli paper "Decision Feedback and Feedforward Receiver (for rates faster than Nyquist's)" in CSELT Rapporti tecnici, Vol. 4, No. 2, pages 97-105, June 1976. Thus, a designer must strike a compromise between the expense of providing enough cells to equalize for the worse case circuit conditions and the range of circuits desired to be equalized with a given equalizer. The Tamburelli equalizer appears not to be adaptive as to at least the precursor equalization
Linear adaptive equalizers employing transversal filters are, of course, known in the art; but they usually require complex, and hence costly, circuitry for real time updating of tap coefficients and for multiplication of coefficients with the respective tap signals. The high cost flows at least in part from the fact that multibit value representations must be multiplied together,and that is relatively costly. Furthermore, such equalizers are typically used in conjunction with an input signal sampling circuit that samples arbitrarily in a certain time phase, e.g., the center of each bit time, without regard for whether or not that is the time of maximum eye opening. Consequently, signal detection error rates are often much less than optimum. Multiple samples per bit time are also employed to obey the nyquist criterion and share the pulse, but that involves the complexity and cost of dealing with more samples per bit time.
A linear equalizer approach to both precursor and postcursor equalization is shown by H. Kobayashi et al. in U.S. Pat. No. 3,792,356. Here input signal from a channel is reshaped by equalization and coupled to a circuit output. The input signal is applied through a precursor equalizer and a summing circuit to a main equalizer, the output of which, after quantization, is fed back to the summing circuit to be additively combined with the precursor equalizer output. In one embodiment, the precursor equalization function is incorporated into at least one additional tap in the main equalizer.
Another linear equalizer is shown by an E.D. Gibson, U.S. Pat. No. 3,697,689. Here a fine timing recovery system for high speed data transmission systems couples receiver input signals from a circuit of known impulse response through a transversal equalizer to be reshaped before outputting. It is necessary that the impulse response be essentially symmetrical with respect to the main tap. Gibson utilizes adjusted tap signals on either side of the main tap to derive a signal indicative of the difference between the two tap signals to control the phase of a clock signal used in the receiver to a phase of essentially zero difference. Sampling is then performed midway between the phases corresponding to the two taps.
A paper "Towards a Single Chip ISDN transmission Unit" by K. J. Wonda et al., appeared in Proceedings ISSLS '86 at pages 250-255. The authors consider a linear equalizer with a two-tap, fixed-coefficient, filter to produce a signal which, when optimized forces the precursor to zero at the sampling point. The Wouda et al. timing control is responsive to the precursor effect and independent of data signal main cursor amplitude.
An Ehrenbard et al., U.S. Pat. No. 4,494,242 exercises sampling phase control by comparing signal samples at precursor and main cursor times and using the result to control sampling phase. The ratio of the precursor tap to the main cursor tap is chosen on a compromise basis to accommodate the channel response of the set of subscriber loops for the particular digital system under consideration. The sampling time is aligned to coincide with zero precursor instant induced by the multiplication of the received signal by the precursor top and then subtracting this result from the received signal that received one sample time previous.